Assessing VAT errors
Unless HMRC can prove dishonest declarations they cannot assess for VAT errors more than four years after the end of the prescribed accounting period. So if an error is discovered on 31 December 2016, for it to be assessed it must have arisen no earlier than the return for the quarter ended 31 December 2012.
Once HMRC have sufficient evidence of the fact the error must be assessed within one year, otherwise HMRC can only go back two years.
The four year cap does not extend to late registrations where the HMRC are entitled to go back 20 years.